Method and apparatus for controlling dispersion of molten metal in a mold cavity

ABSTRACT

A method and apparatus for controlling the dispersion of molten metal in a mold cavity is disclosed, the control facilitated by a localized densification of foam in a lost foam casting operation for producing metal castings, wherein the filling of regions of the mold cavity which do not lend themselves to castability is maximized, an amount of backfill and casting defects are minimized, and a flow pattern of molten metal and material properties of the resulting casting are optimized.

CROSS REFERENCE OF RELATED APPLICATION

This is a Divisional of U.S. Ser. No. 11/100,049 filed Apr. 6, 2005.

FIELD OF THE INVENTION

The invention relates to lost foam casting and more particularly to amethod and apparatus for controlling dispersion of molten metal in amold cavity in a lost foam casting process for producing metal castings,wherein the control is facilitated by a localized densification of thefoam.

BACKGROUND OF THE INVENTION

A so-called “lost-foam” casting process is a well-known technique forproducing metal castings. A fugitive, pyrolizable, polymeric, foampattern (including casting, gating, runners, and sprue) is covered witha thin (typically in the range of 0.25-0.5 mm), gas-permeable refractorycoating/skin such as mica, silica, alumina, or alumina-silicate, forexample. The pattern is embedded in compacted, unbonded sand to form amold cavity within the sand. Molten metal is then introduced into themold cavity to melt, pyrolyze, and displace the pattern with moltenmetal.

Gaseous and liquid decomposition/pyrolysis products escape through thegas-permeable, refractory skin and into the interstices between theunbonded sand particles. The casting rate or rate at which the moltenmetal enters the mold cavity is limited by the rate the advancing moltenmetal front can displace the pattern from the cavity. This is affectedby the thickness and permeability of the refractory skin/coating.Typical fugitive polymeric foam patterns comprise expanded polystyrenefoam (EPS) for aluminum castings and copolymers ofpolymethylmethacrylate (PMMA) and EPS for iron and steel castings, forexample.

The polymeric foam pattern is made by injecting pre-expanded polymerbeads into a pattern mold to impart the desired shape to the pattern.For example, raw EPS beads (typically 0.2 to 0.5 mm in diameter)containing a blowing/expanding agent (e.g. n-pentane) are: (1) first,pre-expanded at a temperature above the softening temperature ofpolystyrene and the boiling point of the blowing agent; and (2) moldedinto the desired configuration in a steam-heated pattern mold whichfurther expands the beads to fill the pattern mold. Complex patterns andpattern assemblies are made by molding several individual mold segments,and then joining the mold segments by gluing, for example, to form thepattern or pattern assembly.

The molten metal may be either gravity-cast meaning poured from anoverhead ladle or furnace, or countergravity-cast. In gravity-castlost-foam processes, the metallostatic head of the molten metal in thesprue and pouring basin is the driving force for filling the mold cavitywith molten metal. Countergravity-cast lost-foam processes involvecausing the molten metal to flow upwardly by vacuum or low pressure intothe mold cavity from an underlying vessel such as a furnace, forexample.

Gravity-cast, lost-foam processes are known that top-fill the moldcavity by pouring the molten metal into a basin overlying the pattern sothat the molten metal flows downwardly into the mold cavity through agating system located above the pattern. Other gravity-cast methodsbottom-fill the mold cavity by pouring the molten metal into a verticalsprue that lies adjacent the pattern. The sprue extends from above themold cavity to below the mold cavity for filling the mold cavity througha gating system located beneath the pattern so that the molten metalflows vertically upwardly into the mold. Additionally, gravity-castmethods can side-fill the mold cavity by pouring the molten metal into apattern that forms a vertical sprue which lies adjacent the mold cavity.The vertical sprue communicates with the mold cavity via a plurality ofvertically aligned runners and gates which horizontally fill the moldcavity from the side. The vertical sprue may be flanked by two or moremold cavities for making multiple castings with a single pour.

Molten metal flow in a lost foam mold is related to the density of thefoam pattern. Casting engineers are often challenged with a partconfiguration which does not lend itself to castability. Features suchas long straight rails cause metal to flow through a mold quickly whilecausing other areas to back-fill. The back-fill areas can be subject todefects such as folds. Computer simulation programs have been used toattempt to adjust gate area and location in an attempt to optimize flowpatterns.

It would be desirable to develop a method and apparatus for controllingdispersion of molten metal in a mold cavity for a lost foam castingprocess wherein the filling of regions of the mold cavity which do notlend themselves to castability is maximized, an amount of backfill andother casting defects are minimized, and a flow pattern of molten metaland material properties of the resulting casting are optimized.

SUMMARY OF THE INVENTION

Consistent and consonant with the present invention, a method andapparatus for controlling dispersion of molten metal in a mold cavityfor a lost foam casting process wherein the filling of regions of themold cavity which do not lend themselves to castability is maximized, anamount of backfill and other casting defects are minimized, and a flowpattern of molten metal and material properties of the resulting castingare optimized, has surprisingly been discovered.

In one embodiment, the apparatus for locally densifying a lost-foamcasting pattern for controlling dispersion of molten metal in a moldcavity comprises a pattern die with a cylinder and a pattern formingcavity formed therein, the cylinder in communication with the patternforming cavity; a squeeze pin slidably disposed in the cylinder of thepattern die; means for applying a force on the squeeze pin, the meansfor applying a force causing the squeeze pin to slidably move in thecylinder of the pattern die in a direction towards the pattern formingcavity of the pattern die; and a pattern disposed in the pattern formingcavity of the pattern die, wherein a sliding of the squeeze pin causes alocalized densification of the pattern.

The invention also provides methods for controlling dispersion of moltenmetal in a mold cavity.

In one embodiment, the method of controlling a dispersion of moltenmetal in a mold cavity comprises the steps of providing a pattern havingat least one locally densified portion; embedding the pattern in sand toform a mold cavity therein; and introducing molten metal into the moldcavity, wherein the densified portion facilitates a diversion of themolten metal throughout the mold cavity upstream of the densifiedportion of the pattern to promote a complete filling of the mold cavity.

In another embodiment, the method of controlling a dispersion of moltenmetal in a mold cavity comprises the steps of providing a pattern diehaving at least one squeeze pin slidably disposed therein; providing afoam pattern disposed within a pattern forming cavity of the patterndie; compressing a portion of the foam pattern with the squeeze pin todensify a portion of the foam pattern; embedding the foam pattern insand to form a mold cavity therein; and introducing molten metal intothe mold cavity, wherein the densified portion of the foam patternfacilitates a diversion of the molten metal throughout the mold cavityupstream of the densified portion of the foam pattern to promote acomplete filling of the mold cavity.

DESCRIPTION OF THE DRAWINGS

The above, as well as other advantages of the present invention, willbecome readily apparent to those skilled in the art from the followingdetailed description of a preferred embodiment when considered in thelight of the accompanying drawings in which:

FIG. 1 is a schematic sectional view of a lost foam pattern dieincluding foam densification means according to an embodiment of theinvention, and showing the die during a foam filling step;

FIG. 2 is a schematic sectional view of the lost foam pattern dieillustrated in FIG. 1 showing the die during a foam densification step;

FIG. 3 is a schematic sectional view of a lost foam pattern disposed insand and showing the pattern during initial stages of a molten metalfilling step;

FIG. 4 is a schematic sectional view of the lost foam patternillustrated in FIG. 3 showing the pattern during the molten metalfilling step and prior to metal penetration of the densified foam; and

FIG. 5 is a schematic sectional view of the lost foam patternillustrated in FIGS. 3 and 4 showing the pattern during the molten metalfilling step and after metal penetration of the densified foam.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description and appended drawings describe andillustrate various exemplary embodiments of the invention. Thedescription and drawings serve to enable one skilled in the art to makeand use the invention, and are not intended to limit the scope of theinvention in any manner. In respect of the process disclosed and theflow diagrams illustrated, the steps presented are exemplary in nature,and thus, the order of the steps is not necessary or critical.

FIG. 1 depicts a lost foam pattern die 10 according to an embodiment ofthe invention. The die 10 includes a pattern forming cavity 12 formedtherein. The cavity 12 has a shape substantially the same as a desiredcast part (not shown). A cylinder 14 is formed in the die 10 and is incommunication with the cavity 12. Cylinder as used herein is meant tomean a through-hole, cavity or other chamber adapted to have a slidingmember disposed therein.

A squeeze pin 16 is slidably disposed in the cylinder 14. As usedherein, pin is meant to include a piston, plug, or other member which isslidable within the cylinder 14. A hydraulic cylinder 18 is adapted toapply a pressure or force to the pin 16 to cause the sliding of the pin16 within the cylinder 14. It is understood that other means forapplying a force can be used such as a rack and pinion gear set,pressurized air, and a spring, for example.

The position and orientation of the pin 16 illustrated in FIG. 1 createsa pocket 22 in the cylinder 14 in communication with the cavity 12 andfacilitates a filling of the cavity 12 and the pocket 22 with foam beads20. The foam beads 20 may be an expandable polystyrene plastic, forexample. As used herein, pocket is meant to include a cavity, a chamber,or other volume which can be filled with the foam beads 20. A heatsource (not shown) is adapted to apply heat to the die 10.

In order to form a foam pattern 24, the beads 20 are blown or otherwiseconveyed or caused to enter the cavity 12 until the cavity 12 issubstantially filled with the beads 20. Heat is applied to the die 10 bythe heat source, thereby causing the beads 20 to expand and “melt”together to form the foam pattern 24 as shown in FIG. 2. Although foamhas been used herein to form the pattern for exemplary purposes, it isunderstood that other materials having similar properties can be usedwithout departing from the scope and spirit of the invention.

After the foam pattern 24 has been formed, the pin 16 is caused to slidewithin the cylinder 14 in the direction shown in FIG. 2. The movement ofthe pin 16 causes a local compression of the portion of the foam pattern24 disposed in the pocket 22. As a result, a locally densified portion26 is created in the foam pattern 24. It is understood that the foambeads 20 can also be compressed prior to the heating step to result information of the locally densified portion 26. Once the foam pattern 24has been formed, the foam pattern 24 is removed from the die 10 andcoated with a gas-permeable refractory skin (not shown) such as mica,silica, alumina, or alumina-silicate, for example.

The coated foam pattern 24 is embedded in compacted, unbonded sand 28 asshown in FIG. 3. The foam pattern 24 forms a mold cavity 30 within thesand 28. Molten metal 32 is then introduced into the mold cavity 30 tomelt, pyrolyze, and displace the foam pattern 24 with the molten metal32. Gaseous and liquid decomposition or pyrolysis products (not shown)are permitted to escape through the gas-permeable refractory skin andinto the foam pattern 24. The decomposition products then pass throughthe sand 28. Interstices between the unbonded particles of sand 28permit the decomposition products to pass therethrough.

The rate at which the molten metal 32 enters and travels though the moldcavity 30 is limited by the rate the front of advancing molten metal 32can displace the foam pattern 24 from the mold cavity 30. Thus, when themolten metal 32 reaches the densified portion 26 as shown in FIG. 4, theadvancement of the molten metal 32 through the remainder of the moldcavity 30 is delayed, impeded, or slowed. The slowed advancement of themolten metal 32 through the remainder of the mold cavity 30 results inand facilitates the diversion of molten metal 32 throughout the moldcavity 30 and to all areas or sections of the mold cavity 30 upstream ofthe densified portion 26. Thus, complete filling of the mold cavity 30is promoted.

Once the molten metal 32 melts, pyrolyzes, and displaces the densifiedportion 26, the molten metal 32 is permitted to travel normally thoughthe mold cavity 30, as illustrated in FIG. 5. It is understood that aplurality of densified portions 26 can be used as desired to promotecomplete filling of the mold cavity 30. Computer simulation programs canbe used to determine locations of the densified portions 26 in anattempt to optimize flow patterns of the molten metal 32 through themold cavity 30.

It is understood that other methods of local densification of the foampattern 24 can be used. One such method involves producing a pluralityof foam patterns 24 of different densities. The plurality of foampatterns 24 are then bonded together to form a single foam pattern 24representing a desired final shape and configuration of the casting. Theplurality of foam patterns can be bonded together using any conventionalmeans such as gluing, heat welding, or other bonding method as desired,for example. The foam pattern 24 is embedded in sand 28.

Numerous advantages result from the method and apparatus of theinvention described herein. The advantages include a minimization ofcasting defects such as backfill. Additionally, voids in the resultantcasting are minimized, since complete filling of the mold cavity 30including runners and the like is promoted. These advantages, andothers, result in an overall reduction in scrap produced.

From the foregoing description, one ordinarily skilled in the art caneasily ascertain the essential characteristics of this invention and,without departing from the spirit and scope thereof, can make variouschanges and modifications to the invention to adapt it to various usagesand conditions.

1. An apparatus for locally densifying a lost-foam casting pattern forcontrolling dispersion of molten metal in a mold cavity comprising: apattern die with a cylinder and a pattern forming cavity formed therein,the cylinder in communication with the pattern forming cavity; a squeezepin slidably disposed in the cylinder of said pattern die; means forapplying a force on said squeeze pin, said means for applying a forcecausing said squeeze pin to slidably move in the cylinder of saidpattern die in a direction towards the pattern forming cavity of saidpattern die; and a pattern disposed in the pattern forming cavity ofsaid pattern die, wherein a sliding of said squeeze pin causes alocalized densification of said pattern.
 2. The apparatus according toclaim 1, wherein said means for applying a force is a hydrauliccylinder.
 3. The apparatus according to claim 1, wherein said means forapplying a force is a spring.
 4. The apparatus according to claim 1,wherein said pattern is produced from a foam material.
 5. The apparatusaccording to claim 4, wherein the foam material is an expandedpolystyrene foam.
 6. The apparatus according to claim 1, wherein whensaid pattern is embedded in sand to produce a mold cavity therein, thelocal densification facilitates a diversion of a molten metal introducedinto the mold cavity, the diversion promoting a filling of the moldcavity.